Various devices (equipment) can be connected to the telephone line and receive operating power from the line. When the telephone line is the on-hook state, the line voltage is typically higher than in the off-hook state. It would therefore be expected that a device connected to the telephone line would draw more current during the on-hook state than in the off-hook state, in the absence of a voltage regulator connected between the device and the telephone line. However, in some cases, the amount of current drawn by the voltage regulator can exceed a prescribed standard.
It will be recognized that only a restricted amount of current can be drawn from the line while the on-hook condition of the line is maintained. If too much current is drawn, e.g. By the regulator, the on-hook state will be undesirably exited and the off-hook state will be entered. For this reason, the regulatory agencies and/or the telephone companies prescribe a standard that restricts the amount of current that can be drawn from the line in the on-hook line condition.
The line voltage is not uncommonly as low as under 3 volts DC at the end of a line from 24 or 50 volts provided by a PBX or central office, as high as 240 volts AC provided by ringing generators, to as high as 800 volts of a voltage transient. A voltage regulator must be able use any of this wide range of voltages and to withstand the highest voltage and deliver a stable DC voltage to equipment, e.g. 3.3 volts.
In order to conserve power, some telephone regulatory agencies have decreed that the standby, (or on-hook) power drawn from the line power supply equipment, e.g. typically located at a PBX or central office, must be very low, such as less than 25 .mu.A with an applied line tip/ring voltage of 25 volts, and preferably less. In the past, this has been very difficult to achieve.